Use new xkbcommon API for consumed modifiers

Summary:
This change removes the workaround added with
421824b6.

Xkbcommon 0.7 introduces the needed API to make this work without a
workaround.

Test Plan: Test case still passes

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3809